High stress spinning of polyester industrial yarns has during the past decade produced yarns which have achieved a remarkable commercial success. These yarns which have been referred to as high modulus, low shrinkage (HMLS) or dimensionally stable polyester (DSP) yarns, have become the preferred polyester yarn for reinforcement of various articles including tires, V-belts, hoses and the like. These yarns have many performance advantages. For example, in tires, sidewall undulation can be reduced and ride and performance can be improved. Moreover, the yarns have improved work loss characteristics such that less heat is generated by repeated stretching and relaxation of the yarns.
High stress spun polyester industrial yarns and processes for their production was first disclosed in U.S. Pat. No. 4,101,525 to Davis, et al. and 4,195,052 to Davis, et al., both of which are hereby incorporated by reference in the present specification. The polyester yarns and processes for their production, as disclosed in the Davis, et al. patents have substantially improved polyester reinforced products. In addition, U.S. Pat. No. 4,414,169 to McClary discloses an improved process for producing polyester high stress spun and in-line drawn yarns employing improved processing conditions. This patent is also hereby incorporated in the present specification by reference.
In general, the HMLS yarns are produced by melt spinning high intrinsic viscosity polyethylene terephthalate polymer under conditions of high stress so that the as-spun yarn has a birefringence of greater than about 9.times.10.sup.-3 and thereafter drawing the high birefringence as-spun yarn to change or develop yarn physical properties, e.g., strength, modulus and shrinkage.
During the drawing process, the yarns are highly susceptible to mechanical damage which results in broken filaments and/or loops extending outwardly of the yarn bundle, known in the trade as frays. The broken filaments typically result from attempts to optimize the yarn properties by drawing the yarns to a draw ratio which is close to the maximum achievable for the particular yarn. In general, it is known that as the spun birefringence of the yarns is increased, the maximum draw ratio achievable for the particular yarn will decrease; however, a high as-spun birefringence is desirable for improving the stability of the internal yarn structure as explained more fully in the Davis, et al. and McClary patents.
As compared to the "conventional" industrial yarn products, the HMLS products generally may have a lower strength (measured as tenacity). Because the yarns have lower strength than the prior industrial polyester yarns the typical drawing process is often operated close to the maximum obtainable to achieve the highest strength and/or to optimize other properties, which in turn, results in the potential for an increase in the number of frays as discussed earlier. Although the frays can be reduced by lowering the severity of drawing conditions, the consequence can be a lower strength product or a product lacking other optimum properties.
Numerous process modifications have been proposed for improving the yarn tenacity, i.e. strength, and/or decreasing mechanical damage that is, reducing the number of frays in the drawn yarn. Such proposals have included improving the process uniformity and the uniformity of polymer used for spinning the yarns. In addition, it has been proposed to improve the process by improving heating control (for example at the spinneret), uniformity of quench, finish type and application and improving the quality and uniformity of the drawing and winding process. In general, however, success has been limited.
U.S. Pat. No. 4,514,350 to Roth, et al. and U.S. Pat. No. 4,605,364 to Roth, et al. disclose a method and apparatus for melt spinning polyester filaments wherein molten polymer is extruded through at least two rows of different size orifices in a single spinneret and thereafter subjected to quenching. The orifices nearer the quench source have a larger diameter than the orifices furtherest from the quench source. Fibers were spun according to the patent from a 0.62 IV polymer through orifices having diameters ranging from 0.009 in up to 0.010 in. The resultant filaments have a substantially decreased birefringence variability within the filament bundle and the denier variability among the filaments is substantially increased, which can improve the uniformity of dye uptake in the resultant filament.
In the case of high stress spun polyester industrial fibers, many of conventional process modifications do not effect the desired changes in the final product and in other cases, are not fully applicable or effective because of the high stress imposed upon the molten polymer streams during the high stress spinning stage and/or the significant drawing forces applied to the filaments in subsequent drawing stages.